Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies.

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Photovoltaics

Solar Water heating

Wind Energy

Solar Thermal

Solar thermal technologies can be used for water heating (including pool and spa), space heating, space cooling and process heat generation.

Solar Water Heating

Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40 degrees) from 60 to 70% of the domestic hot water use with temperatures up to 60 °C can be provided by solar heating systems.The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools.

China is the world leader in their deployment with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020. Israel and Cypress are the per capita leaders in the use of solar hot water systems with over 90% of homes using them. In the United States, Canada and Australia heating swimming pools is the dominant application of solar hot water with an installed capacity of 18 GW as of 2005.

Solar Energy for Heating, Cooling and Ventilation

In the United States, HVACheating, ventilation and air conditioning (HVAC) systems account for 30% (4.65 EJ) of the energy used in commercial buildings and nearly 50% (10.1 EJ) of the energy used in residential buildings. Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy.

Radiant Floors

Geothermal Heat Pump

Solar Air Heater

Thermal mass is any material that can be used to store heat—heat from the Sun in the case of solar energy. Common thermal mass materials include stone, cement and water. Historically they have been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night. However they can be used in cold temperate areas to maintain warmth as well. The size and placement of thermal mass depend on several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment.

A solar chimney (or thermal chimney, in this context) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. Performance can be improved by using glazing and thermal mass materials in a way that mimics greenhouses.

Deciduous trees and plants have been promoted as a means of controlling solar heating and cooling. When planted on the southern side of a building, their leaves provide shade during the summer, while the bare limbs allow light to pass during the winter. Since bare, leafless trees shade 1/3 to 1/2 of incident solar radiation, there is a balance between the benefits of summer shading and the corresponding loss of winter heating. In climates with significant heating loads, deciduous trees should not be planted on the southern side of a building because they will interfere with winter solar availability. They can, however, be used on the east and west sides to provide a degree of summer shading without appreciably affecting winter solar gain.

Solar Energy for Process Heat

Solar concentrating technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial system was the Solar Total Energy Project (STEP) in Shenandoah, Georgia, USA where a field of 114 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory. This grid-connected cogeneration system provided 400 kW of electricity plus thermal energy in the form of 401 kW steam and 468 kW chilled water, and had a one hour peak load thermal storage.

Evaporation ponds are shallow pools that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams.

Clothes lines, clotheshorses, and clothes racks dry clothes through evaporation by wind and sunlight without consuming electricity or gas. In some states of the United States legislation protects the "right to dry" clothes.

Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22 °C and deliver outlet temperatures of 45–60 °C. The short payback period of transpired collectors (3 to 12 years) makes them a more cost-effective alternative than glazed collection systems. As of 2003, over 80 systems with a combined collector area of 35,000 Square metrem² had been installed worldwide, including an 860 m² collector in Costa Rica used for drying coffee beans and a 1,300 m² collector in Coimbatore, India used for drying marigolds.

Solar Electricity

Sunlight can be converted into electricity using photovoltaics (PV), concentrating solar power (CSP), and various experimental technologies. PV has mainly been used to power small and medium-sized applications, from the calculator powered by a single solar cell to off-grid homes powered by a photovoltaic array. For large-scale generation, CSP plants like SEGS have been the norm but recently multi-megawatt PV plants are becoming common. Completed in 2007, the 14 MW power station in Clark County, NevadaClark County, Nevada and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger photovoltaic power stations in the US and Europe.

Photovoltaics (PV)

A solar cell, or photovoltaic cell (PV), is a device that converts light into direct current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in the 1880s.Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%.

The earliest significant application of solar cells was as a back-up power source to the Vanguard I satellite, which allowed it to continue transmitting for over a year after its chemical battery was exhausted. The successful operation of solar cells on this mission was duplicated in many other Soviet UnionSoviet and USAAmerican satellites, and by the late 1960s, PV had become the established source of power for them. Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar, and they remain vital to the telecommunications infrastructure today.

The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without power gridgrid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, Buoynavigational buoys and railroad crossings. These off-the-gridoff-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004.

The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s. Economies of scale which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 USD/watt in 1971 to 7 USD/watt in 1985. Steadily 1980s oil glutfalling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax credits associated with the Energy Tax Act of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996.

Since the mid-1990s, leadership in the PV sector has shifted from the US to Japan and Germany. Between 1992 and 1994 Japan increased R&D funding, established net metering guidelines, and introduced a subsidy program to encourage the installation of residential PV systems. As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999. Germany has become the leading PV market worldwide since revising its Feed-in tariffs in GermanyFeed-in tariff system as part of the Renewable Energy Sources Act. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007. Spain has become the third largest PV market after adopting a similar feed-in tariff structure in 2004, while France, Italy, South Korea and the US have seen rapid growth recently due to various incentive programs and local market conditions.

Concentrating Solar Power (CSP)

Concentrated sunlight has been used to perform useful tasks since the time of ancient China. A legend claims that Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse, SicilySyracuse. Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine in 1866, and subsequent developments led to the use of concentrating solar-powered devices.

Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage. A solar power tower concentrates sunlight from a field of heliostats on a central tower. A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. Trough systems provide the best land-use factor of any solar technology.

A parabolic dish system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. Parabolic dish systems give the highest efficiency among CSP technologies.

The stirling solar dish combines a parabolic concentrating dish
with a Stirling engine stirling heat engine which normally drives an electric generator. The advantages of stirling solar over photovoltaic cells are higher efficiency of converting sunlight into electricity and longer lifetime. A stirling engine has an approximate mean time before failure (MTBF) of 25 years.

A solar power tower uses an array of tracking reflectors (heliostats) to concentrate light on a central receiver atop a tower. Power towers are less advanced than trough systems but offer higher efficiency and better energy storage capability. The Solar Two in Barstow, California and the PS10 solar power towerPlanta Solar 10 in Sanlucar la Mayor, Spain are representatives of this technology.

Energy Storage Methods

Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, storage media or back-up power systems must be used.

Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal thermal storeseasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well-designed systems can lower peak demand, shift time-of-use to off-peakoff-peak hours and reduce overall heating and cooling requirements.

Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity. With grid-tied systems, excess electricity can be sent to the transmission Grid-tied electrical systemgrid. Net metering programs give these systems a credit for the electricity they deliver to the grid. This credit offsets electricity provided from the grid when the system cannot meet demand, effectively using the grid as a storage mechanism.

Pumped-storage hydroelectricity stores energy in the form of water pumped when energy is available from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water to run through a hydroelectric power generator.

Solar Energy Development, Deployment and Economics

Between 1970 and 1983 photovoltaic installations grew rapidly, but falling oil prices in the early 1980s moderated the growth of PV from 1984 to 1996. Since 1997, PV development has accelerated due to supply issues with oil and natural gas, global warming concerns (see Kyoto Protocol), and the improving economic position of PV relative to other energy technologies.FactdateAugust 2008 Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007. Since 2006 it has been economical for investors to install photovoltaics for free in return for a long term power purchase agreement. 50% of commercial systems were installed in this manner in 2007 and it is expected that 90% will by 2009.

Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999.

Commercial concentrating solar power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a LEC of 12–14 ¢/kWh.

Solar installations in recent years have also largely begun to expand into residential areas, with governments offering incentive programs to make "green" energy a more economically viable option. In Canada the government offers the RESOP (Renewable Energy Standard Offer Program). The program allows residential homeowners with solar panel installations to sell the energy they produce back to the grid (i.e., the government) at 41¢/kWh, while drawing power from the grid at an average rate of 20¢/kWh (see feed-in tariff). The program is designed to help promote the government's green agenda and lower the strain often placed on the energy grid at peak hours. With the incentives offered by the program the average payback period for a residential solar installation (sized between 1.3 kW and 5 kW) is estimated at 18 to 23 years, considering such cost factors as parts, installation and maintenance, as well as the average energy production of a system on an annual basis.

Daniel Lincot, the chairman of the 2008 European Photovoltaic Solar Energy Conference and the research director of the Paris-based Photovoltaic Energy Development and Research Institute, said that photovoltaics can cover all the world energy demand. Photovoltaics are 85 times as efficient as growing corn for ethanol.